Electrochemical ChlorineFree AC Disinfection of Water Contaminated With Salmonella Typhimurium...

ISSN 1023�1935, Russian Journal of Electrochemistry, 2010, Vol. 46, No. 3, pp. 306–311. © Pleiades Publishing, Ltd., 2010.Original Russian Text © N.N. Barashkov, D. Eiseinberg, S. Eisenberg, G.Sh. Shegebaeva, I.S. Irgibaeva, I.I. Barashkova, 2010, published in Elektrokhimiya, 2010, Vol. 46, No. 3,pp. 320–325.

306

INTRODUCTION

Sewage water contains numerous microorganisms.There are numerous methods to reduce the number ofinfection components to an acceptable level for drain�age to any water pools [1]. Earlier, the chlorinationmethod was traditionally used; in recent years, ozonetreatment, ultraviolet irradiation, and various electro�chemical processes are applied. Selection of the disin�fection method significantly depends upon the cost ofrequired energy, especially in the cases, where largequantities of water should be treated.

Salmonella Typhimurium (S. Typhimurium) bacte�ria are classified as the most dangerous microorgan�isms causing serious intestinal disorders of humans.Food products of animal and vegetable origin are ofteninfected with S. Typhimurium bacteria. Water remain�ing after poultry washing at poultry farms (poultrywater) is also often infected with these bacteria.Recently, poultry water was disinfected by such chem�ical methods as chlorination [2], ozonation [3], andothers [4]. The disadvantages of these methodsinclude potential formation and accumulation of toxicchemical products as, for example, in the case of chlo�rination or high cost as in the case of ozonation [5].

Electrochemical treatment of water, includingpoultry water became interesting relatively recently

[1, 6–12]. The authors of [1] found that effective elec�trolytic water treatment using NaCl electrolyte isaccompanied by generating significant quantities ofelementary chlorine, hypochlorite, and, probably,chlorate and requires a high initial concentration ofNaCl (over 0.15%). At such concentration, chlorideanions cause corrosion of any bimetal parts of watersystems, for example, water distributors, cooling tow�ers, etc. Replacement of chloride anions with phos�phates, nitrates, sulfates, or carbonates, which do notcause corrosion, is more favorable with respect to theequipment operation costs and increases profitabilityof the electrochemical water sterilization process.

The mechanism of electrochemical disinfectiondepends upon numerous parameters, including thenature of electrolyte. The authors of [6, 13, 14] calcu�lated the number of S. Typhimurium bacteria in NaCl,Na2SO4, and Na3PO4 solutions subjected to low�volt�age and low�current electrolysis. Already in five�min�utes, this treatment was shown to reduce the concen�tration of live S. Typhimurium bacteria by one or twoorders of magnitude.

The authors of [15�17] studied the electrochemicaldisinfection mechanism for E. coli B bacteria in NaCl,NaNO3, and Na2SO4 solutions. The disinfection effi�ciency of electrolysis was close to that of ozonation,i.e., much higher than that of ordinary chlorination.Electrochemical disinfection was shown to kill bacte�

Electrochemical Chlorine�Free AC Disinfection of Water Contaminated with Salmonella Typhimurium Bacteria

N. N. Barashkova, D. Eiseinberga, S. Eisenberga, G. Sh. Shegebaevaa, I. S. Irgibaevab, and I. I. Barashkovac, z

aMicrotracers Corporation, 1370, Van Dike Avenue, San Francisco, 94124, USAbGumilev Eurasian National University, 5, ul. Munaitpasova, Astana, 010008, Kazakhstan

cSemenov Institute of Chemical Physics, Russian Academy of Sciences, 4, ul. Kosygina, Moscow, 119991, RussiaReceived March 30, 2009

Abstract—Deionized (DI) water contaminated with Salmonella Typhimurium (S. Typhimurium) bacteriawas disinfected by alternating current. Ammonium sulfate was used as electrolyte. Disinfection was carriedout in the circulation system including an electrochemical cell with stainless steel electrodes. The processefficiency was estimated and the number of killed bacteria was directly proportional to the water treatmenttime and concentration of hydroxyl radicals generated by electrolysis. The presence of OH radicals wasdetected with N,N�dimethyl�p�nitrosoaniline (RNO) used as a spin trap. Similar experiments were carriedout with water remaining after poultry washing at poultry farms and additionally contaminated with S. Typh�imurium bacteria. Measures were recommended to increase the process efficiency and decrease the watertreatment time.

Key words: water disinfection with alternating current, electrochemical treatment, hydroxyl radicals, spintrap, kinetic model

DOI: 10.1134/S1023193510030079

z Corresponding author: [email protected] (I.I. Barashkova).

RUSSIAN JOURNAL OF ELECTROCHEMISTRY Vol. 46 No. 3 2010

ELECTROCHEMICAL CHLORINE�FREE AC DISINFECTION OF WATER 307

ria not by electric field. In the opinion of the authorsof [18], the major contribution to the disinfectioneffect is made by generation of hydroxyl radicals. Theformation mechanism of OH radicals in electrolysisof aqueous solutions was discussed by the authors of[19–21]. They showed, in particular, that the gener�ally accepted schemes of anodic oxidation andcathodic reduction are simplified for the reactions:

2Н2О = 4Н+ + 4е + О2,

4Н2О + 4е = 2Н2 + 4ОН–

Formation of atomic hydrogen (H*) is one of theintermediate stages in the course of molecular hydro�gen formation in the reaction of cathodic reduction.Here a part of atomic hydrogen initiates the followingsequence of reactions:

Н* + О2 = НО2, НО2 + Н* = Н2О2.

In the cases, where electrodes are made of metal (M),hydrogen peroxide takes part in the reaction generat�ing hydroxyl radicals:

Н2О2 + М2+ = М3+ + •OH + ОН–.The life time of hydroxyl radicals is too short for theiridentification by ESR method. Nevertheless, an effec�tive method for detection of OH radicals was suggestedby the authors of [21]. They analyzed the interactionproducts of OH radicals with tert�butylnitron in elec�trolysis of potassium chlorate solutions. The generatedlong�living radicals were detected by ESR method.

In addition to OH radicals, the disinfection processmay involve also other intermediate highly active com�

pounds such as anion radicals. In the case of sul�fate electrolyte [22], there may be persulfate and sul�fate radicals. In electrolysis of phosphate buffer[23, 24], bacteria are killed due to formation of hydro�gen peroxide.

Our earlier studies [25, 26] represent the resultsfrom electrochemical disinfection of deionized (DI)water contaminated with E. coli B and containing(NH4)2SO4 as electrolyte. We suggested a kineticmodel for elimination of bacteria and a quantitativemethod describing the rate of interaction betweenhydroxyl radicals and bacteria and considered theeffects of such parameters as treatment time, electro�lyte concentration, and initial concentration of bacte�ria. In this work, we analyzed the results of a similarstudy carried out for DI water and poultry water con�taminated with a high concentration of S. Typhimu�rium bacteria.

EXPERIMENTAL

We used N,N�dimethyl�p�nitrosoaniline (RNO)used as a spin trap for hydroxyl radicals. The concen�tration of OH radicals resulting from electrolysis wasestimated with a spectrophotometer by the change inthe RNO optical absorption spectral intensity with thepeak at 440 nm. The solutions of DI water contained

O• –

2

the (NH4)2SO4 electrolyte concentration of 0.025 to0.5% and RNO concentration of 0.0027 to 0.003%(1.8 × 10–6 to 2.0 × 10–6 M). Our selection of(NH4)2SO4 as electrolyte was accounted for by the factthat it is relatively harmless and cheap. The poultrywater was provided by the experimental poultry farmat the University of Arkansas (Fayetteville, Arkansas,USA). Before electrolysis, this water was diluted withthe same volume of DI water containing the necessaryquantities of (NH4)2SO4 and RNO. According to theresults of microbiological analysis, the population ofS. Typhimurium bacteria in the prepared solution wasbelow the detection level. We prepared the solutions ofS. Typhimurium bacteria with various concentrationsby dilution of water with highly concentrated suspen�sion containing approximately 42 × 106 cfu/ml ofbacteria.

Our experimental water disinfection device is rep�resented in Fig. 1. We found that alternating current of0.21 A (50 Hz frequency) corresponding to the currentdensity of 60 mA/cm2 does not cause corrosion of steelelectrodes in the selected range of ammonium sulfateconcentrations. We maintained the constant currentdensity by selecting the initial voltage in the rangefrom 40 to 170 V with respect to the electrolyte con�centration.

Before starting each experiment, the laboratorydevice was disinfected for 20 min with hot water flowat the temperature of 75 to 80°C. The initial tempera�ture of the treated water was 20°C. The water flow ratethrough the electrochemical cell was 40 l/min. Theportions of 10 ml were sampled through the open coverof electrochemical cell and stored at 4°С for microbi�ological analysis. The colonies of S. Typhimurium werecounted after incubation for 48 h at 35°С.

1

2

3

~

Fig. 1. Laboratory electrochemical water disinfectiondevice: (1) pump, (2) water flow meter, (3) plastic electro�chemical cell; parallel set of ten stainless steel net elec�trodes (5.5 cm diameter) with 3 mm intervals.

308


BARASHKOV et al.

RESULTS AND DISCUSSION

DI water contaminated with S. Typhimurium

Figure 2 represents a typical curve of live bacteriaconcentration in solution vs. electric current treat�ment time. Our analysis of experimental data enablesto make to conclusions. First, without electric current,no addition of (NH4)2SO4 and RNO to DI water con�taminated with S. Typhimurium causes any significantbactericidal effect. Second, if electric current isapplied, the number of live bacteria vs. treatment timecan be described with a linear logarithmic curve. Todescribe the process of electrolysis, we used a modifiedkinetic model designed for the disinfection of naturalwater contaminated with coliforms [15].

According to this model, the initial number of bac�teria (n0) and the current number of bacteria (n) attime moment t are interrelated as follows:

= – kt, (1)

were k is the factor depending on the current densityfor the constant volume of water to be disinfected andconstant surface area of the electrodes.

The diagram in Fig. 2 enables to calculate factor kand to determine such essential parameter of disinfec�tion process as the minimum time td, which is neces�sary to decrease the concentration of bacteria to n = 1.With respect to calculation of the bacteria populationin the selected dimension, such low concentrationshould be considered as negligibly small. Therefore, tdcan be referred to as the time required for completeelimination of bacteria. It corresponds to the intersec�tion point of the experimental line with the time axis.Table 1 represents the parameters of k and td calculatedfor each concentration of electrolyte.

The contaminated water flow through the electro�chemical cell without the applied voltage slightlydecreases the population of bacteria, which becomesnoticeable after 30 min experiment. A similar effectwas earlier noted for DI water contaminated withE. coli B [26] and seems to be accounted for by theknown biocide effect of cavitation to microorganisms.

Poultry water contaminated with S. Typhimurium

Our electrochemical experiments with poultrywater contaminated with S. Typhimurium bacteriawere carried out under the same conditions as ourexperiments with DI water. First, as in the case of DIwater, electrochemical disinfection of poultry watertakes place far more effectively than water flowthrough electrodes under no voltage. Second, the livebacteria count vs. water treatment time is describedwith logarithmic curve.

Compare factor k and time td for poultry water withthe corresponding data for DI water in Table 1. We can

nlog n0log

7

5

4

3

2

log

n [c

fu/m

l]

80604020

43

21

6

0Time, min

Fig. 2. Live S. Typhimurium bacteria count vs. disinfectiontime in (NH4)2SO4 solutions with concentrations of0.025% (1, 2) and 0.05% (3, 4) without electrolysis (1, 3)and with electrolysis (2, 4).

Table 1. Factor k and complete disinfection time td calculated for linear model (equation (1)) for electrolysis of aqueous S. Typh�imurium solutions

Electrolyte concentration S. Typhimurium disinfection media logn0 [cfu/ml] k td, min

0.025 DI water 6.01 0.0499 120.2

poultry water 5.76 0.0448 128.6

0.05 DI water 6.06 0.0550 110.1

poultry water 5.68 0.0464 122.3

0.2 DI water 6.13 0.0565 108.5

poultry water 5.63 0.0472 119.2

0.5 DI water 6.21 0.0634 98.0

poultry water 5.59 0.0503 111.2



see that achievement of complete disinfection by elec�trochemical treatment of S. Typhimurium bacteria inpoultry water requires more time than in the case of DIwater.

Kinetics of interaction between OH radicals and S. Typhimurium bacteria

Generation of OH radicals by electrochemicaltreatment of contaminated water was successfully usedfor oxidation of organic compounds, for example,phenols [28]. The advantages of RNO used as a spintrap are accounted for by high selectivity of the reac�tion of interaction between RNO and OH radicals.Consider the reactions of OH radicals spending in thecourse of electrolysis. The interaction scheme can berepresented as follows:

•OH + spin trap spin adduct.Figure 3 shows the change in the absorption spectra

of RNO in (NH4)2SO4 aqueous solution contaminatedwith S. Typhimurium under the conditions of electrol�ysis. As we see, a solution flow through the zero�cur�rent electrolytic cell only slightly decreases the opticaldensity at 440 nm. Under the conditions of electroly�sis, the optical density changes rapidly. It decreases farmore rapidly if electrolysis takes place in the bacteria�free solution. These results apparently evidence for�mation of OH radicals.

We neglect potential occurrence of other reactionsinvolving hydroxyl radicals, excepting elimination ofbacteria and interaction with RNO. Then expenditureof OH radicals in electrochemical treatment can bedescribed as follows [26]:

•OH + RNO R�(HO)NO•,

•OH + E. coli B dead bacteria. We use the kinetic model suggested for description

of two competing reactions [28]:

1/Gt = 1/Gо{1 + [B])/(kOH[RNO])}, (2)

where Gt is the RNO decoloration rate in the presenceof bacteria, Go is the RNO decoloration rate in theabsence of bacteria, [B] is the concentration of bacte�ria in solution, [RNO] is the RNO concentration insolution, and and kOH are the relevant reactionrate constants.

Like in the case of DI water, voltage applied topoultry water sharply decreases the RNO optical den�sity at 440 nm. Compare the kinetic characteristics ofRNO interaction with hydroxyl radicals in DI waterand poultry water. For this calculation, we selected theelectrolyte concentration of 0.2% and various initialconcentrations of S. Typhimurium (Table 2). TheRNO concentration was measured after 40 min treat�ment of water. The 1/Gt vs. [B]/[RNO] is a linear rela�

tionship with the slope of 1/Go /kOH). The radi�

cal�bacteria interaction rate constant was calcu�

kOH

kOH'

(kOH'

kOH'

(kOH'

kOH'

lated in the assumption that the RNO�OH radicalreaction rate constant is 1.2 × 1010 mol–1 s–1 [4]. Thuscalculated values of for DI water and poultrywater are represented in Table 2 along with the valuesof constant k and time td for both kinds of water as cal�culated by equation (1). We see that the bacteria elim�ination rate in DI water is higher than that in poultrywater.

Therefore, disinfection of poultry water is a slowerprocess than disinfection of DI water. This conclusionfollows from comparison of the bacteria eliminationrate and agrees with the determined time td

required for complete disinfection of water. As followsfrom [29], the bacteria elimination rate under electro�chemical treatment of S. Typhimurium suspensions is

kOH'

kOH'

0.16

0.12

0.08

0.04

0480440400

12

3

4

5

(c)

0.16

0.12

0.08

0.04

0

1

234

5

(b)

0.16

0.12

0.08

0.04

0

1

5

(a)

Opt

ical

den

sity

, a.u

.

Wavelength, nm

Fig. 3. Absorption spectra of RNO in DI water contaminatedwith S. Typhimurium before (1) and after flow through elec�trochemical cell for 5 (2), 20 (3), 40 (4), 60 (5) min; watertreated without voltage (a), under voltage (b), under volt�age in bacteria�free solution (c).

310


BARASHKOV et al.

higher in DI water than that in poultry water. This dif�ference was accounted for by the fact that poultrywater contains additionally blood cells, fat micelles,and other organic substances, which can coat the cellsof bacteria and thus protect them from the effect ofradicals. Such explanation seems to be true also forinterpretation of the results represented in this work.

Comparison of interaction kinetics of hydroxyl radicals with S. Typhimurium bacteria

Table 3 represents the results from electrochemicaltreatment of DI water contaminated with bacteria oftwo species. All the measurements were carried outwith the same electrolyte in the same concentrationunder the same conditions of AC treatment. Appar�ently, for approximately the same initial concentrationof bacteria (deviation not exceeding 3%), completedisinfection in the case of E. coli B needs less time thanin the case of

S. Typhimurium. Our conclusion that disinfectionof S. Typhimurium is a slower process as compared tothat of E. coli B is confirmed with the bacteria elimi�nation rate constants These constants were cal�

culated in [26] as 3.91 × 106 and 6.01 × 106 cfu s–1,respectively.

The efficiency of electrochemical disinfection canbe enhanced also by varying such parameters as watervolume and flow rate through electrochemical cell aswell by changing the nature of the electrode materials.The energy loss of this process decrease at a lower elec�tric resistance of the electrolyte solution, for example,for a shorter distance between electrodes, larger sur�face area, or higher electrolyte concentration. Theprocess cost can be essentially decreased by selecting

kOH' .

the electrolyte, for example, if (NH4)2SO4 is replacedwith cheaper Na2SO4.

CONCLUSIONS

(1) Chlorine�free AC electrochemical disinfectionwith ammonium sulfate electrolyte is an effectivemethod for disinfection of DI water contaminatedwith S. Typhimurium bacteria.

(2) If all other conditions are the same, disinfectionof poultry water requires more time than disinfectionof DI water.

(3) Use of N,N�dimethyl�p�nitrosoaniline as aspin trap experimentally confirmed formation ofhydroxyl radicals in electrolysis of aqueous ammo�nium sulfate solutions. Hydroxyl radicals make anessential impact to the bactericidal process.

(4) Under the same electrolytic conditions, disin�fection of DI water contaminated with S. Typhimu�rium bacteria takes more time than in the case of DIwater contaminated with E. coli B. This conclusion isconfirmed with the kinetic calculations for expendi�ture of hydroxyl radicals.

ACKNOWLEDGEMENTS

The authors are grateful to L. Lam for microbio�logical tests of bacteria concentration.

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Table 2. Factor k and complete disinfection time td calcu�lated by equation (1) and S. Typhimurium elimination rateconstants calculated by equation (2)

S. Typhimuri�um disinfec�tion media

logn0 [cfu/ml]

k td, min,

cfu s–1

DI water 6.13 0.057 109 3.9 × 106

DI water 6.32 0.055 116

DI water 6.43 0.053 122

DI water 6.85 0.055 125

poultry water 5.63 0.047 119 1.7 × 106

poultry water 5.76 0.047 124



kOH'

Table 3. Complete water disinfection time td for various ini�tial concentrations of S. Typhimurium and E. coli B bacteria

Electrolyte concentration

Bacteria species in DI water* logn0 td, min

0.025 S. typhimurium 6.01 120

E. coli B 6.12 103


E. coli B 6.10 96


E. coli B 5.95 89


E. coli B 6.23 89.3

* Disinfection data of DI water contaminated with E. coli B takenfrom [20].



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Electrochemical ChlorineFree AC Disinfection of Water Contaminated With Salmonella Typhimurium...

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